1. Field of the Invention
The present invention relates to an organic electroluminescent device.
2. Description of the Related Art
In recent years, light emitting apparatuses utilizing organic electroluminescent devices (organic EL devices) have been developed. FIG. 6 schematically shows the configuration of an organic EL device 1. On a substrate 2 made of glass or the like, an anode 3, organic EL layers 8 (hole transport layer 4, light emitting layer 5 and electron transport layer 6), a cathode 7 and the like are formed in layers. In the drawing, partitions, insulating films, sealing members and the like are not shown. The two electrodes 3 and 7 are connected to external lines through the lead lines (terminals) of the electrodes. When an electric field is applied to the electrodes, holes and electrons recombine in the light emitting layer 5 in the region sandwiched between the electrodes 3 and 7 to emit light.
In the case of producing a display apparatus capable of color display, for example, an anode is formed on a substrate in stripes, and then patterning is performed using organic EL materials such that organic EL layers corresponding to the colors red (R), green (G) and blue (B), respectively, are shown repeatedly on the anode. Subsequently, a cathode is formed on the organic EL layers, and the terminals (externally connecting terminals) of each of the electrodes are connected to external lines such as control lines, signal lines or the like. Thereby, organic EL devices corresponding to RGB are arranged to form pixels, and thus color display may be carried out.
A material constituting the light emitting layer is required to satisfy conditions such as good luminescence efficiency, good carrier transportability and good film forming properties. The material may be composed only of a light emitting material, or may also be composed of a mixture of a light emitting material and a host material which does not emit light and has charge transportability.
Furthermore, there have been proposed luminescent devices developed under an aim of improving the luminescence efficiency or increasing the life, by allowing the light emitting layer to have a concentration gradient of the constituent material (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-155862, JP-A No. 2001-189193, JP-A No. 2004-6102 and JP-A No. 2002-313583). However, in practice, an organic EL device achieving improvements in the luminescence efficiency and durability, by allowing the entire light emitting layer to emit light by precisely controlling the carrier balance in the light emitting layer, is not found.